The France Under-21 striker did not manage to find the net in any of his 14 appearances last season, having joined from Auxerre on a free transfer but seeing his progress hampered by a back problem.

However he produced a clinical display on Saturday and Costa Rica forward Joel Campbell, a man expected to be given his Premier League break in 2014/15 after three seasons out on loan, was also on target as he swept home a fine volley to pull Arsene Wenger’s men out of sight before the interval.

It took 26 minutes for the deadlock to be broken at Emirates Stadium, as Aaron Ramsey span into space to square for Sanogo to flick home, but the floodgates soon opened.

Five minutes before half-time, Campbell kept his cool to caress the ball into the bottom corner, before Sanogo completed his hat-trick by netting twice in the space of as many minutes as he first rolled into an empty net and then prodded past Artur after being picked out from the left by Kieran Gibbs.

Four minutes after the re-start and the 21-year-old Frenchman had his fourth, with Artur spilling a low drive from Ramsey into his path.

Benfica pulled one back just past the hour mark, with a long throw from Maxi Pereira touched on for Nicolas Gaitan to nod in from close range, but Arsenal were able to ease over the line and hand big-money signing Alexis Sanchez his debut on a productive afternoon.

Wenger had named a mixed side, which included teenager Calum Chambers, the £16million signing from Southampton, lining up in defence along with Spanish youngster Hector Bellerin, while the likes of Sanchez, France full-back Mathieu Debuchy - purchased from Newcastle - and England midfielder Jack Wilshere on all the bench.

Arsenal's German World Cup winning trio Per Mertesacker, Lukas Podolski and Mesut Ozil are not scheduled to be involved until after the FA Community Shield against Manchester City at Wembley next weekend, with Crystal Palace the first opponents when the action starts for real on August 16.

Benfica made a bright start, with Gaitan floating a shot onto the top of the crossbar, with reserve goalkeeper Damian Martinez beaten.

Campbell spooned a volley wide from Ramsey's chipped through ball and then Sanogo stabbed a low cross from Bellerin past the near post.

Sanogo, however, did find his range on 26 minutes, flicking the ball past Moraes Artur after Ramsey had spun into the Benfica half and swept down the right flank.

Benfica rallied briefly, but were 2-0 down on 40 minutes. This time it was 19-year-old right-back Bellerin, on loan at Watford for a spell last season, who scampered away, before exchanging a neat one-two with Sanogo. He then picked out Campbell arriving at pace into the left side of the penalty area, and he drilled a fine left-foot volley into the bottom corner.

Arsenal extended their lead on the stroke of half-time, with Sanogo knocking in a pass from Campbell as the Benfica defence pushed up and called for offside.

Sanogo completed his hat-trick in first-half stoppage time, hooking the ball home from Gibbs' left-wing cross.

The former Auxerre forward - whose career in France was interrupted by a broken leg - chalked up his fourth at the start of the second half, slotting past Artur after Ramsey's shot was saved.

Following substitutions by both teams, Benfica netted a consolation goal on 61 minutes, Gaitan heading in after Arsenal failed to deal with a long throw.

Chile forward Sanchez, signed from Barcelona, was introduced to rapturous applause when he replaced Campbell with 15 minutes left.

Benfica captain Maxi Pereira struck the post, as did Arsenal substitute Francis Coquelin late on from Sanchez's cut-back as the closing stages were played out at a low tempo.

In the earlier kick-off, Monaco drew 2-2 with Valencia. Colombia striker Falcao, who missed the World Cup because of a serious knee injury, made an appearance off the bench for Monaco in the closing stages, his first since January.

Portugal Under-21 centre-back Ruben Vezo diverted an Abdennour header into his own net to give Monaco a 20th-minute lead, but former Chelsea and Real Madrid defender Ricardo Carvalho scored an own goal himself from Antonio Barragan's cross to level up the scoreline eight minutes before the interval.

After the break, former Liverpool loanee Aly Cissokho set up Paco Alcacer in the 70th minute to put the La Liga side in front, but Monaco substitute Lucas Ocampos drilled in an equaliser 10 minutes later.

till I suppose it still gives him at least another year in the job. By the time it comes around , he can always make up another excuse. First it was 3 months , then a year .....Next year ? Whats his excuse with Van Persie ?